The invention relates to a hydraulic clutch which is dependent on rotation speed difference and comprises a rotating housing, a hydrostatic displacement machine arranged in this housing, a shaft, a friction clutch for connecting the shaft to the housing, and a piston, in which case, when a difference in rotation speed occurs between the housing and the shaft, a pressure which acts on the piston is produced in a compression chamber. This pressure is applied to the friction surfaces of the friction clutch.
Such clutches are used in various arrangements in the drive run of motor vehicles, either to transmit torque directly or to block a connected differential transmission, for driving the wheels on one axle or for distributing the torque between two axles. In any case, the hydrostatic displacement machine comprises an inner rotor and an outer rotor, which move relative to one another if there is any rotation speed difference between the inner rotor and the outer rotor or, in particular, between the housing and the shaft and thus, in the compression chamber, exert on the piston the pressure which is required to act on the clutch.
An apparatus of this generic type is disclosed, for example, in WO 96/41090. There, the arrangement is designed such that a valve is fitted in the piston, and closes when a specific pressure is reached. Oil thus enters the clutch area only until the clutch is engaged. The oil is then simply centrifuged away in some manner through openings on the circumference of the rotating housing. When the clutch starts to engage, that is to say it produces a large amount of friction heat, the oil is still absent. The clutch plates thus become severely heated during driving, and are subject to high wear.
Since the weight of modern motor vehicle drive runs is designed to be optimized, it is necessary to limit the transmitted torque, and this can be achieved by limiting the pressure in the compression chamber. In the hydraulic clutch which is dependent on rotation speed difference according to DE 41 09 789 C2, this is achieved by means of an overpressure valve accommodated in the stationary housing. However, since the compression chamber is located in the rotating part, a rotating bushing is required to connect the valve and compression chamber. This makes the design complex, particularly if the clutch is intended to operate without any external control action. However, the overpressure valve must then be accommodated in the rotating parts.
As a rule, overpressure valves comprise a spring-loaded valve body. If this is fitted in a rotating part, then it is subject to the centrifugal force. Furthermore, such valves require a relatively large physical space, particularly if they are aligned with the rotation axis.
Although U.S. Pat. No. 5,611,746 relating to such items discloses a constriction valve in the rotating part, this can, however, only be set permanently and is thus not spring-loaded and, furthermore, is connected only indirectly to the compression chamber; and not to the clutch area at all. In addition, it could not be spring-loaded at all, since the centrifugal force would act against the spring if aligned radially, and would produce a friction brake if aligned axially. Nevertheless, this constriction valve is difficult to accommodate, see FIGS. 1 and 9 in this respect.
The object of the invention is thus to improve an apparatus of type discussed above such that the cooling of the friction clutch is improved, the space required is minimized, and it is possible to limit the torque in a very simple manner without any disturbing centrifugal force influences.